Public health signifi cance
Our fi ndings on the epidemiology of HIV in MSM have many implications for HIV prevention, treatment, and care. The second
101
and third
reports in this Series will address these implications in detail and propose a targeted set of interventions for prevention for these men. First, the high transmission probability, high force of infection, and the potent eff ect of prevalent HIV infections in networks clearly suggest that interventions to reduce infectiousness, such as HAART for HIV-positive MSM, will probably be essential to achieve control of these epidemics. Our modelling outputs suggest that even substantial behaviour change, such as reductions in extraprimary partnerships, would not reduce transmission frequency enough to control HIV incidence in MSM networks. Interventions that reduced the probability of acquisition for men engaging in unprotected receptive anal intercourse, such as oral pre-exposure prophylaxis, rectal microbicide, or successful treatment for prevalent HIV infections, will probably be key. The high burden of infections in MSM demands more vigorous and scientifi cally informed responses, and the development and implementation of strategies to address the high biological risks associated with anal sex. The molecular epidemiology of HIV in MSM also has several important implications for public health. An HIV vaccine based on subtype B (assuming subtypes, as currently understood, have roles in vaccine design) would have wide applicability in Europe, North and South America, and parts of Asia. This reality has already been of proven use to HIV vaccine trials. The CRF01_AE component, proposed for trials in MSM in Thailand, would protect against an additional and substantial proportion of infections in MSM in Asia. The apparent clustering of transmission events in MSM, the speed with which HIV can move through MSM networks, and the reported high heterogeneity of HIV in acute infections in MSM might be a challenge to HIV vaccines for this route of exposure. That daily oral pre-exposure prophylaxis with a combination of emtricitabine and tenofovir disoproxil fumarate is so far the only biomedical intervention with evidence for effi cacy in MSM